Topic Six - Propagation, Architectural Acoustics, Resonance, & Standing Waves

Question 1

What are the different ways material absorbs different frequencies by different amounts?*

Answer 1

Some examples of absorbent materials are:
- Acoustic traps (the sound wave is forced to make multiple collisions, with each collision attenuating the sound)
- Curtains

Some examples of reflective surfaces are:
- Concrete
- metal

Question 2

What is reverberation?*

Answer 2

Reverberation is the persistence of sound after the source has stopped, caused by multiple reflections from surfaces like walls, ceilings, and floors. It creates a prolonged sound instead of an immediate stop.

Question 3

What is reverberation time and how is it measured?*

Answer 3

Reverberation time (RT 60) is how long the sound takes to drop by 60 dB.

Question 4

How can the the acoustic properties of a space be modeled and altered with different materials?*

Answer 4

Software allows the creation of 3D models of rooms to be created to estimate their acoustic properties.

Absorption coefficients can then be assigned to each surface and object.

Allows us to see how the sound propagates

Question 5

What is the transmission loss and what factors influence the degree of transmission loss?*

Answer 5

Transition loss is how much sound is lost through an object.

It is determined by the mass law:

TL = 20log₁₀ (tpπf/p₀c₀)

t= thickness of the barrier (m)
p = density of the barrier (kg/m2)
f = frequency (Hz)

If any of these increase (t,p,f) there is more transition loss

Question 6

What is a sound field?

Answer 6

A space in which a sound wave propagates

Question 7

What is a free field?

Answer 7

• A sound field with no obstacles
• The wave propagates from a source uniformly in all directions

Question 8

What is the audible frequncy range for humans?

Answer 8

20 Hz to 20 KHz

Question 9

How does the wavelength impact the wave’s interaction with obstacles?

Answer 9

If a wave encounters an object that is small relative to its wavelength, the wave will be transmitted with little to no change.

If a wave encounters an object that is large relative to its wavelength, the wave will be attenuated.

An example of a small wave length to large object occurs with the “head shadow effect”. This helps us with directional hearing across the horizontal plane (inter-aural difference).

Question 10

True / false: when a wave hits a surface, the angle of reflection is always greater than its angle of incidence.

Answer 10

False, the angle of incidence and angle of reflection are both the same

Question 11

How is sound reflected on a convex surface?

Answer 11

The sound waves are dispersed.

Question 12

How is sound reflected on a concave surface?

Answer 12

The sound is focused and creates a region of intensity

Question 13

How is sound reflected on right-angled corners?

Answer 13

It returns to its origin

Question 14

True/false: reverberation is always bad and we want to have it minimized in all environments

Answer 14

False, in some situations reverberation is good and in others it is bad.

In highly reverberant environments, speech intelligibility can be reduced.

• Normal listeners ok up to 1 second reverberation time
• Hearing impaired listeners below 0.4-0.5 seconds

Question 15

How is sound absorption measured?

Answer 15

Coefficients of sound absorption tell us how much an object absorbs sound.

• An absorption coefficient of 1 = total absorption
• An absorption coefficient of 0 = total reflection

The unit of absorption is in sabins

Absorption coefficients are measured at multiple frequencies.

Question 16

What elements are needed to form a simple harmonic oscillator?

Answer 16

A mass (m) and a linear spring (k)

Question 17

For a linear oscillator, when displacement is at its peak, what is its velocity and acceleration?

Answer 17

Velocity is at 0
Acceleration is at maximum

Question 18

For a linear oscillator, when displacement is at zero, what is its velocity and acceleration?

Answer 18

Velocity is at maximum
Acceleration is at 0

Question 19

True/false: resonators are filters

Answer 19

True - resonators take a broadband range of frequencies and filter it into a specific frequency (e.g., blowing into a empty bottle)

Question 20

How can you change a resonant frequency?

Answer 20

• The larger the mass, the lower the resonant frequency
• The larger the stiffness, the higher the resonant frequency

Question 21

True/false: Resonant systems cannot be made to resonant outside of their natural resonant frequency

Answer 21

False, they can be made to resonant outside of their natural resonant frequency.

Question 22

At frequencies below the resonant frequency, the impedance of the system is dominated by…

Answer 22

Stiffness

Question 23

At frequencies above the resonant frequency, the impedance of the system is dominated by…

Answer 23

Mass

Question 24

When mass and stiffness equal out at resonant frequency, what is the maximum velocity limited by?

Answer 24

Friction

Question 25

What is a Q-factor

Answer 25

Q-factor is a measure of how good of a resonator a given system is. If an object has a high q-factor, it is very sharply tuned, will resonant for a long time If an object has a low q-factor, it is broadly tuned and will not resonant for long. Calculated by: Resonant frequency / bandwidth (drop 3dB from resonant frequency and minus the two points to get the bandwidth)

Question 26

What is dampening?

Answer 26

Damping is any loss of energy in an oscillating system that causes it to lose amplitude over time. Two common forms of damping are energy loss to heat, and energy transfer to other systems.

Question 27

What is a Helmholtz resonator?

Answer 27

A resonator formed by two masses of air, one of which acts as a spring

Question 28

What happens when waves of the same frequency from different sources are combined?

Answer 28

Their superposition will either cause constructive or de-constructive interference

Question 29

What is a beat?

Answer 29

Beats are created by the superposition of two waves of the similar amplitude, but slightly different frequencies. The beat frequency is equal to the difference in the frequencies of components (e.g., wave 1 = 100 Hz, wave 2 = 110 Hz; beat frequency = 10)

Question 30

What is a standing wave?

Answer 30

A standing wave is created by the superposition of two waves of the same frequency and similar amplitude which are traveling in opposite directions They have regions of minimum displacement = nodes and regions of maximum displacement = antinodes

Question 31

In a wave with a fixed end, will the reflection be in the opposite phase or in phase?

Answer 31

Fixed end = opposite phase Free end = in phase This applies for both complex and simple waves

Question 32

What are two ways to reduce standing waves in a test environment?

Answer 32

- Reduce reflections - Use of warble tones

Question 33

How does sound reflect in a closed and open end pipes?

Answer 33

In a closed end pipe: - Pressure antinodes (high pressure) - Velocity node (low velocity) In an open end pipe: - Pressure node (low pressure) - Velocity antinode (high velocity)

Question 34

What type of 1/4 wavelengths are present in closed and open pipes?

Answer 34

Closed = odd number of 1/4 wavelengths Open = Even number of 1/4 wavelengths